The stimulation frequency has been found to affect the axial symptoms of freezing of gait (FOG) and speech in advanced Parkinson's disease (PD) patients with subthalamic nucleus deep brain stimulation (STN DBS), with the traditionally high stimulation frequency of 130 Hz worsening while low frequency of 60 Hz improving these symptoms. The effect of stimulation frequency on another important axial symptom, swallowing dysfunction, has not been studied. The investigators hypothesize that stimulation frequency could have similar effect on swallowing function.